1. Field of the Invention
The present invention relates to a fixing device, and more specifically relates to a fixing device of the thermal type for use in copying apparatus, laser printers and the like which use an electrophotographic process.
2. Description of the Related Art
Copying apparatus, laser printers and the like which use an electrophotographic process typically are provided with a fixing device for fusing a toner image onto a paper sheet. Such fixing devices generally comprise a fixing roller having an internal heater, and a pressure roller disposed so as to confront said fixing roller. The fixing roller receives the heat generated by the internal heater to elevate the temperature to a predetermined temperature (about 150 degrees C.), whereas the pressure roller is heated via the fixing roller. Temperature fluctuation of fixing devices having the aforesaid construction are described below with reference to FIG. 23.
In the drawing, reference numeral 2301 refers to the temperature change on the surface of the fixing roller. When the printing operation of a laser printer and the like is started and an electric current is applied to the heater, the temperature of the fixing roller is elevated to about 150 degrees C. (t1) and the temperature of the pressure roller is also elevated by receiving the heat of the fixing roller. At the moment the leading edge of a fed sheet arrives between the aforesaid rollers (t2), the surface temperature of the pressure roller has been elevated to about 100 degrees C. Then, the toner image on the surface of the paper sheet is fused via the heat from the fixing roller and the pressure roller so as to be fixed thereon.
The amount of heat maintained by the pressure roller during the passage of the sheet between the rollers (t2-t3) is affected by the paper and the toner, such that the surface temperature of the pressure roller is reduced. When the trailing edge of the paper sheet is removed from between the two rollers (t3), the pressure roller is again heated by the fixing roller, such that the surface temperature of the pressure roller is again elevated (t3-t4).
The fixed strength of the fixed toner is shown in FIG. 24. This drawing shows the fixed strength of the toner on each part of the paper sheet relative to the transport direction. As previously mentioned, sufficient heat is applied to the leading half of the paper sheet (in the sheet transport direction), and the toner penetrates into the fibers of the paper. Therefore, the fixed strength (reference numeral 2401 in the drawing) of the toner is excellent in the leading half of the paper. However, since the temperature of the pressure roller is subject to a drop due to the loss of the heat of the pressure roller, such that the toner on the trailing half of the paper sheet is inadequately fused to the sheet on the trailing half of said sheet. Therefore, the fixed strength of the toner is insufficient in the trailing half of the paper sheet (reference numeral 2402 in the drawing), and the toner readily separates from said sheet and thereby results in the disadvantage of poor toner adhesion. Such a disadvantage is common in fixing devices provided with a pressure roller of small diameter and/or slight heating capacity.
Although the poor toner adhesion on the trailing half of the paper sheet may be eliminated by raising the temperature of the fixing roller, such a remedy produces new disadvantages such as high temperature offset and the like because the toner on the leading half of the sheet is overheated.
That is, in conventional fixing devices, the total amount of heat applied to the leading half of the paper sheet by the fixing roller and the pressure roller, and the total amount of heat applied to the trailing half of the sheet by the fixing roller and the pressure roller are different. Thus, there is a problem in adequately fixing the toner along the entirety of the leading half and the trailing half of the sheet.
Furthermore, because the pressure roller temperature is raised via the heat of the fixing roller, the moisture contained in the paper sheet is vaporized via said heat applied by the fixing roller when the paper passes through the aforesaid fixing device. The vaporization of the moisture produces a curling of the ends of the paper sheet, as shown in FIG. 25. The curl in the center portion of the paper sheet is only slightly apparent since the curl is straighten due to own weight of the ends of said sheet. While the fixing temperature may be reduced so as to correct the aforesaid curl, the toner fixed strength is reduced thereby, producing a new disadvantage inasmuch as the toner readily separates from the paper.
When a laser printer is in the wait state, the fixing roller 2610 and the pressure roller 2620 are stopped, as shown in FIG. 26. In order to shorten the time for raising the temperature when a printing operation starts, the temperature of the fixing roller 2610 is maintained at 100 to 150 degrees C. During the wait period, therefore, the nip portion of the pressure roller 2620 (the portion of contact between the fixing roller 2610 and the pressure roller 2620) becomes an area of high temperature. In the drawing, the area of the dashed lines of the pressure roller 2620 indicates the high temperature area. The portion of the pressure roller 2620 heated by the fixing roller 2610 in the wait state is called the high temperature area hereinafter.
When the laser printer starts a printing operation, the fixing roller 2610 and the pressure roller 2620 are rotated, but even after the pressure roller 2620 begins to rotate the high temperature area of said pressure roller 2620 remains higher in temperature than other parts of said roller. As shown in FIG. 27, when the high temperature area of the pressure roller 2620 forms the nip portion, the leading edge of the paper sheet (the leading edge of the sheet in the paper transport direction) reaches the nip portion, and the leading edge of the paper in contact with the high temperature area on the circumference of the pressure roller 2620 becomes higher in temperature than other parts of the paper sheet. Thus, the moisture contained in the leading edge part of the paper is excessively vaporized, and a curl is produced in the leading edge.
FIG. 28 is a graph showing the change in temperature in the nip portion of the pressure roller 2620. When the pressure roller 2620 starts to rotate simultaneously with the start of a printing operation and completes a single rotation, the high temperature area of the pressure roller 2620 again arrives at the nip portion. The previously described curl is similarly produced at the moment at which the high temperature area of the pressure roller 2620 which has rotated m times arrives at the nip portion (time t1) and the leading edge of the paper sheet is gripped between the rollers at the nip portion. When the center part of the paper sheet comes into contact with the high temperature area of the pressure roller 2620, this center part of the paper becomes higher in temperature than other parts of the paper yet the aforesaid curl is only slightly apparent. As previously described, the curl in the center portion of the paper sheet is straightened due to the weight of both ends of the paper.